change pivot-center in image. Video Editiing
I watched the video, I think the rotation happens always using the center of the VSE video output. In the video the overlay image position is such that its lower left corner happens to be roughly at the overall frame center, so rotation seems to happen pivoting on its lower left corner,but is just a coincidence. 

Edit: The image is not rotated itself. The image has just an offset that moves it to the upper right corned. Another (green) "trasformation" strip is applied to the image, and that green strip is what is rotated.

Her is what I think it happens: